Briefly, this invention has to do with a method of making a plug-in fuse assembly like that disclosed in U.S. Pat. No. 3,909,767, granted Sept. 30, 1975, and which preferably comprises a plug-in element including a coplanar plate-like body of fuse metal having a pair of laterally spaced coplanar terminal forming blade portions to be received by pressure clip terminals in a mounting panel, coplanar current carrying extensions at the inner end portions of the pair of terminal forming blade portions and a fuse-forming link portion of reduced thickness interconnecting the current carrying extensions, and a synthetic plastic housing, which can be comprised of two confronting housing halves which can be assembled or snapped over the plug-in element, but which is preferably a one piece housing which is open at the inwardly facing side thereof, and which contains the coplanar plate-like body of the plug-in element with the current carrying extensions and the interconnecting fuse-forming link portion thereof of reduced thickness within the housing and with the pair of terminal forming blade portions thereof extending outwardly from the housing and preferably through the opening therein.
Generally, the method of making such a plug-in fuse assembly comprises providing a blank of fuse metal which is blanked or stamped to provide the pair of laterally spaced coplanar terminal forming blade portions which are interconnected by a transverse relatively rigid web, the coplanar current carrying extensions thereof and the relatively fragile interconnecting fuse-forming link portion of reduced thickness.
The synthetic plastic housing is inserted over said blank of fuse metal, preferably by way of the opening at the inwardly facing side of the one piece synthetic plastic member, with the current carrying extensions and the interconnecting fuse-forming link portion of reduced thickness of the blank preferably within the housing and with the pair of terminal blade portions of the blank, which are interconnected by the relatively rigid transverse web, extending outwardly from the housing element preferably through the opening therein. The blank of fuse metal is suitably secured in the synthetic plastic housing element as by staking or the like.
The exposed transverse web interconnecting the pair of terminal blade portions adds rigidity to the blank and securely maintains the relative positions of the pair of terminal blade portions, the current carrying extensions and the interconnecting fuse-forming link portion of reduced thickness, as the synthetic plastic housing is inserted over and secured to the blank. Thus, distortion, breakage or other damage to the blank is effectively prevented during these operations. The exposed transverse web of fuse metal interconnecting the exposed terminal blade portions of the blank is then blanked or stamped out to complete the formation of the coplanar plate-like body of fuse metal having the pair of laterally spaced coplanar terminal portions and of the plug-in fuse assembly. The housing is secured to the blank so that it acts as a rigid insulating interconnection between the current carrying extensions. Thus, the subsequent severance of the web does not adversely affect the strength of the resulting plug-in fuse element. Also, the outer end portions of the exposed pair of terminal forming blade portions may be coined to form tapers thereon to facilitate insertion thereof into the clip terminals in the mounting panel.
In the aforementioned U.S. Pat. No. 3,909,767 and in the method of the aforementioned co-pending application Ser. No. 610,978, the reduced thickness of the interconnecting fuse-forming link portion is provided by initially milling and squeezing the blank of fuse metal only at a point in the blank where the interconnecting fuse-forming link portion is formed by the subsequent blanking operation. This requires a separate milling and squeezing operation at a precise point in each blank of fuse metal. With this procedure it is extremely difficult to maintain sufficiently accurate tolerances for the reduced thickness of the fuse-forming link portion of the plug-in element, and this is compounded where the blanks form integral parts of a continuous intermittently advanced strip of fuse metal, wherein deviations in reduced thickness can also occur from blank to blank, all of which is detrimental to mass production of plug-in fuse assemblies having accurate fuse ratings. In addition to this problem of maintaining tolerances and accurate fuse ratings there is also the problem of time consumption and cost in such procedure.
In the method of the instant invention and of the aforementioned application Ser. No. 640,253, the aforesaid milling and squeezing operation at precise points in each blank and the aforementioned problems involved therewith are eliminated. Here, in the preferred form of the invention the blank of fuse metal is initially longitudinally provided throughout its length with a continuous central portion of reduced thickness so that, when the blank is blanked, the interconnecting fuse-forming link portion of the blank is of less thickness than the spaced coplanar terminal forming blade portions and the current carrying extensions. Preferably, a continuous strip of fuse metal is utilized which is initially longitudinally provided throughout its length with a continuous central portion of reduced thickness from which the coplanar plate-like bodies of the plug-in elements are to be formed. The continuous central portion of reduced thickness is preferably provided in the continuous strip of fuse metal by controlled conventional continuous milling and scarfing procedures, whereby the tolerances of the continuous central portion of reduced thickness throughout the length of the strip of fuse metal are kept within close limits. As a result, the thicknesses of the fuse-forming link portions of reduced thickness of the coplanar plate-like bodies of the plug-in elements, subsequently blanked from the strip, are maintained within close tolerances, not only in each plug-in element, but, also, from plug-in element to plug-in element blanked from the continuous strip.
The central portion of reduced thickness of the continuous strip or blank of fuse metal may be formed by milling and scarfing. The strip or blank of fuse metal is preferably selectively plated, such as tin plated, along only those portions which form the pair of laterally spaced coplanar terminal forming blade portions and the current carrying extensions thereof to provide, without short circuiting of the fuse-forming link portions thereof, improved electrical contact of the coplanar terminal forming blade portions of the ultimate plug-in element with the pressure clip terminals in the mounting panel in which they are received. (Such selective plating of the strip of fuse metal before it is blanked is an invention of Allen Ciesmier.)
As the strip of fuse metal having the continuous central portion of reduced thickness is sequentially advanced, it is blanked to provide at spaced intervals in the strip longitudinally interconnected blanks, each containing the pair of laterally spaced coplanar terminal forming blade portions which are interconnected by the transverse web, the coplanar current carrying extensions thereof and the interconnecting fuse-forming link portion of reduced thickness involved in the coplanar plate-like body of the plug-in fuse element.
In this connection, the method includes severing the end blank from the sequentially advancing strip of fuse metal and longitudinally inserting over said end blank of the sequentially advancing strip the synthetic plastic housing with the current carrying extensions and the interconnecting fuse-forming link portion of reduced thickness of the end blank within the housing and with the pair of terminal blade portions of the end blank, which are interconnected by the transverse web of the strip, extending outwardly from the housing through the opening therein. The severed end blank of the strip forming an almost completed plug-in fuse element is secured in the housing so the housing forms a rigid support between the current-carrying extensions. As expressed above, the exposed transverse web interconnecting the pair of terminal blade portions temporarily adds rigidity to the blank before and during the placement of the housing over the severed end blank. Since the strip of fuse metal, and, hence, the blanks formed therefrom are provided throughout their lengths with a central portion of reduced thickness, the transverse webs, interconnecting the pairs of terminal blade portions, is also of reduced thickness like the fragile fuse-forming link portion. However, the exposed transverse web has such a considerable width compared to the width of the fuse-forming link portion that it is effective in preventing distortion, breakage or other damage to the blank as it is inserted in and secured to the housing.
In the aforementioned copending application Ser. No. 640,253 the thickness of the central portion of reduced thickness of the blank of fuse metal and, hence, the thickness of the interconnecting fuse-forming link portion of the plug-in element are selectively varied and controlled to provide plug-in fuse assemblies with a given desired wide range of fuse ratings. Thus, a wide range of fuse ratings is accomplished with blanks having widely different thicknesses in the central portions despite the use of a single location and configuration of similar width and/or length dimension of the interconnecting fuse-forming link portions of reduced thickness. Thus, a single blanking die configuration may be utilized for a number of different ratings of the plug-in fuse assemblies. However, this requires the making and stocking of a large inventory of many different fuse metal strips and blanks to be selectively used. In addition to being costly, it is difficult to maintain accurate dimension tolerances in such inventory of many different fuse metal strips and blanks. Furthermore, particularly in the plug-in fuse assemblies of low ampere ratings, the thickness of the fuse metal strips and blanks are quite small which make them fragile and pose problems in handling such strips and blanks and in blanking the blanks to provide the interconnecting fuse-forming link portions therein.
The aforementioned difficulties involved in the method of making the miniature plug-in fuses of the aforementioned copending application Ser. No. 640,253 are eliminated, or at least minimized, by the method of the instant invention, Here, the continuous central portions of reduced thickness of the strips or blanks of fuse metal from which the fuse-forming link portions of both very low and very high current rated plug-in fuse elements are blanked can have a fixed non-fragile thickness dimension. This is achieved by blanking the strips or blanks of fuse metal so that the fuse-forming link portion of fixed reduced thickness dimension have widely varying locations, and/or configurations thereof. Thus, relatively low fuse ratings are obtained by locating the fuse-forming link portions at locations where heat conduction is at a minimum and by giving the same a thin, deeply undulating shape. The highest rated fuse would have a wide straight fuse link located at a point where heat conduction is at a maximum. By so doing, making and stocking of a large inventory of many different fuse metal strips and blanks covering a large range of fuse ratings is eliminated and costs are substantially reduced. Maintenance of accurate dimension tolerances is facilitated by the identically dimensioned strips and blanks. The identically dimensioned strips or blanks are readily handled and may be readily blanked without distortion or damage since the thickness dimensions of the central portions of the reduced thickness of the fuse metal strips and blanks may be maintained at a reasonable thickness affording strength thereto. The interconnecting fuse-forming link portions remain strong and they are not fragile. While separate blanking dies are needed for each fuse rating, the costs thereof constitute initial tooling costs, as distinguished from ongoing material costs involved with the use of many different strips and blanks of fuse metal, so that the total costs for the manufacture of the plug-in fuse assemblies of this invention will be considerably less, particularly on a mass production basis, this in addition to better tolerance control and handling.
Further objects of this invention reside in the particular method steps and in the cooperative relationships between the method steps in making the aforementioned plug-in fuse assembly.
Other objects and advantages of this invention will become apparent to those skilled in the art upon reference to the accompanying specification, claims and drawings.